Field of the Invention
The present invention relates to a position detecting apparatus configured to detect a position of a movable member, in particular, a position detecting apparatus configured to detect a position of a movable member based on a plurality of signals, and to a lens apparatus and an image pickup apparatus including the position detecting apparatus.
Description of the Related Art
As an apparatus for measuring a distance of movement of an object, besides an incremental encoder for measuring a distance of relative movement, there is hitherto known an absolute encoder capable of detecting an absolute position.
Japanese Patent Application Laid-Open No. H08-304113 discloses a Vernier type absolute encoder. The Vernier type absolute encoder disclosed in Japanese Patent Application Laid-Open No. H08-304113 has a configuration including a main track and at least one sub track, which are formed by grid patterns having different pitches. Based on a slight shift between detected signals, which is generated due to a difference in pitch between the tracks, a position in a section over which the Vernier type absolute encoder circulates once is identified (hereinafter also referred to as “absolute position detection” or “absolute position calculation”).
Japanese Patent Application Laid-Open No. H05-45151 discloses timing of switching to an absolute counting operation in the Vernier type absolute encoder. The Vernier type absolute encoder has a configuration in which tracks are formed at smaller pitches and at larger pitches. As the timing of switching to the absolute counting operation based on the tracks described above, when a speed is lowered to a speed at which erroneous detection does not occur in an incremental measurement at the smaller pitches, an operation of the Vernier type absolute encoder is switched to the absolute counting operation.
The above-mentioned related-art absolute encoders are configured to calculate an absolute position based on a slight shift between detected signals, which is generated due to a difference in pitch between the tracks, and hence are sensitive to scratches and motes on a scale. However, although an error can be found during inspection, there is no method of finding an error during absolute position detection.
In particular, when motes adhere to the scale after the inspection, there is a problem in that a different position may be detected erroneously during the absolute position detection.